According to wikipedia, the diameter of natural spider silk is 4µm, which is a cross section area of 1.26e-11 m². Multiplied by the tensile strength of natural spider silk (5.4 GPa) gives the capacity to lift 68 milinewtoms, or 7 Grams!

http://www.wolframalpha.com/input/?i=%284e-6+meter+%2F+2%29%...